This invention relates to polymers of benzocyclobutene monomers having acid functionality and their use in microelectronics applications.
In certain applications in the microelectronics industry, such as interlayer insulators for integrated circuits, interposers, flat panel displays, and multichip modules, bumping redistribution, passivation/stress buffers, and thin film build-up layers on printed circuit boards, having a pattern of a dielectric material is useful or even necessary. Methods of forming this pattern include dry etching, wet etching, screen printing, ink jet printing, and use of dielectric materials which are themselves photoimageable.
In dry etching, a mask, composed of a material which is resistant to the etching process (frequently an O2 plasma), is applied over the dielectric material. A plasma or laser is used to remove portions of the dielectric material which are not protected by the mask. The mask is then removed, leaving a patterned dielectric layer. See for example, Photosensitive Polyimides; Technomic Publishing Company, Inc.; 1995, pp. 254-55.
Similarly, a mask, typically a photoresist, is used in wet etching. The photoresist is applied over the dielectric material and image-wise exposed to activating radiation. The photoresist and the dielectric material are then removed in a corresponding image-wise manner during a developing step (typically with a aqueous base). The remaining photoresist material is subsequently removed, leaving an image-wise distribution of the dielectric material. For examples of this process see e.g. Photosensitive Polyimides; Technomic Publishing Company, Inc.; 1995, pp. 248-253. When the dielectric or dielectric precursor is itself photosensitive, some costs and complexities of the etching methods are avoided. Specifically, there is no need for a mask material and no need to coat, image, and remove the mask material. See for example, Photosensitive Polyimides; Technomic Publishing Company, Inc.; 1995, pp. 258-260. In negative photosensitive systems, the portion of the dielectric material or its precursor which is not exposed to activating radiation is removed during development. In positive systems, the portion of the dielectric material or its precursor which is exposed to activating radiation is removed during development. In other words, positive systems use a dark field mask rather than a light field mask. Positive systems generally have the additional benefit of being less susceptible to contamination than solvent developed negative systems.
Polymers of cyclobutarenes (also referred to herein as benzocyclobutenes) are known to be useful as insulating layers in electrical devices, protective films for semiconductor elements, as passivation films and as photoresists. Negative photosensitive cyclobutarene compositions, developable with a solvent have been developed. See for example, Cyclotene(trademark) Series 4000 available from The Dow Chemical Company; Negative Tone Photodefinable Benzocyclobutene Formulations For Thin Film MicroElectronic Applications, Proceedings of 10th International Conference on Photopolymers, October-November 1994; and WO 96/31805.
However, a cyclobutarene based composition developable in an aqueous base would be highly desirable. Such a composition would be highly useful in both photosensitive methods and wet etch methods for forming a patterned dielectric.
The Inventors have developed a cyclobutarene based polymer system having acid functionality. The materials, upon further cure, have excellent qualities of toughness, adhesion, dielectric constant, and low stress. The preferred system is soluble in an aqueous base and can be used to generate patterned films with excellent resolution without the need to handle organic developer solvents.
Thus, according to a first embodiment, the invention is a curable cyclobutarene based polymer comprising acid functional pendant groups. xe2x80x9cCurable polymerxe2x80x9d as used herein includes polymers that can be further cured or crosslinked as well as oligomers that can be further reacted to form higher molecular weight polymeric materials. Preferably, the acid functional groups are present at equivalent weights of about 200 to about 330 g/mole of acid functionality, more preferably about 220 to about 300 g/mole of acid functionality, and most preferaly about 230 to about 270 g/mole of acid functionality. For the preferred monomer (a) 1,3-bis(2-bicyclo[4.2.0]octa 1,3,5-trien-3-yl ethenyl)-1,1,3,3 tetramethylsiloxane (referred to herein as DVS-bisBCB) and BCB-acrylic acid as made in Example 1, an equivalent weight of less than about 290 g/mole of acid functionality provides solubility in alkaline aqueous solutions. If the amount of acid groups is too low, the material will be insufficiently soluble in aqueous base. If the amount of acid groups becomes too high, water retention by the polymer may become a problem in certain uses or applications of the material.
Preferably, the polymer is the partially polymerized product of monomers comprising
(a) a cyclobutarene monomer having the formula: 
xe2x80x83wherein
B1 is an n-valent organic linking group, preferably comprising ethylenic unsaturation,
Ar1 is a polyvalent aromatic or heteroaromatic group and the carbon atoms of the cyclobutane ring are bonded to adjacent carbon atoms on the same aromatic ring of Ar1;
m is an integer of 1 or more;
n is an integer of 1 or more; and
R1 is a monovalent group;
and (b) a cyclobutarene monomer having a pendant group with acid functionality. Additional monomers, such as acrylate ester functional cyclobutarenes, may also be used. In fact, in effect, a terpolymer can be formed using certain acrylate ester functional cyclobutarene monomers (bxe2x80x2), such as a t-butyl acrylate ester, instead of the acid functional cyclobutarene. Under normal polymerization conditions the t-butyl groups are at least partially eliminated to generate carboxylic acid groups. Alternatively, the acrylate ester functional cyclobutarenes can be at least partially hydrolyzed by conventional methods (Organic Chemsitry, Morrison and Boyd, Allyn and Bacon, N.Y., 1975, 3rd Ed, pp. 675-681) to generate carboxylic acid groups.
According to a second embodiment the invention is a photosensitive composition comprising a curable cyclobutarene based polymer comprising acid functional pendant groups and a photoactive composition which functions as a dissolution inhibitor. Photosensitive as used in this application means that a latent, developable image or an immediately discernible image is formed if the material is exposed to activating wavelengths of radiation.
According to a third embodiment the invention is the use of a curable cyclobutarene based polymer comprising acid functional pendant groups in a wet etch process.
According to a fourth embodiment the invention is the use of a curable cyclobutarene based polymer comprising acid functional pendant groups as an aqueous developable, positive photoresist.
As used herein the term xe2x80x9cgroupxe2x80x9d means the structure as shown or recited with or without reasonable substitution so long as that substitution does not effect the function of the group. In contrast, the term xe2x80x9cmoietyxe2x80x9d means the structure shown with no substitution.
The cyclobutarene monomer (a) has the formula 
xe2x80x83wherein
B1 is an n-valent organic linking group, preferably comprising ethylenic unsaturation, or B1 is absent. Suitable single valent B1 groups preferably have the formula xe2x80x94CR8xe2x95x90CR9Z, wherein R8 and R9 are independently selected from hydrogen, alkyl groups of 1 to 6, most preferably 1 to 3 carbon atoms, and aryl groups, and Z is selected from hydrogen, alkyl groups of 1 to 6 carbon atoms, aryl groups, xe2x80x94CO2R7 wherein R7 is an alkyl group, preferably of up to 6 carbon atoms, an aryl group, an aralkyl group, or an alkaryl group. Most preferably Z is xe2x80x94CO2R7 wherein R7 is an alkyl group, preferably of up to 6 carbon atoms, an aryl group, an aralkyl group, or an alkaryl group. Suitable divalent B1 groups include xe2x80x94(CR8xe2x95x90CR9)oxe2x80x94(Zxe2x80x2)oxe2x88x921, wherein R8 and R9 are as defined previously, o is 1 or 2, and Zxe2x80x2 is an alkyl group of 1 to 6 carbon atoms, an aromatic group, or a siloxane group. Most preferably o is 2 and Zxe2x80x2 is a siloxane group.
Ar1 is a polyvalent aromatic or heteroaromatic group and the carbon atoms of the cyclobutane ring are bonded to adjacent carbon atoms on the same aromatic ring of Ar1, preferably Ar1 is a single aromatic ring;
m is an integer of 1 or more, preferably 1;
n is an integer of 1 or more, preferably 2-4, more preferably 2; and
R1 is a monovalent group, preferably hydrogen, lower alkyl of up to 6 carbon atoms.
The synthesis and properties of these cyclobutarenes, as well as terms used to describe them may be found, for example, in U.S. Pat. Nos. 4,540,763; 4,724,260; 4,783,514; 4,812,588; 4,826,997; 4,999,499; 5,136,069; 5,185,391; 5,243,068 all of which are incorporated herein by reference.
According to one preferred embodiment, the monomer (a) has the formula 
wherein
each R3 is independently an alkyl group of 1-6 carbon atoms, trimethylsilyl, methoxy or chloro; preferably R3 is hydrogen;
each R4 is independently a divalent, ethylenically unsaturated organic group, preferably an alkenyl of 1 to 6 carbons, most preferably xe2x80x94CH2xe2x95x90CH2xe2x80x94;
each R5 is independently hydrogen, an alkyl group of 1 to 6 carbon atoms, cycloalkyl, aralkyl or phenyl; preferably R5 is methyl;
each R6 is independently hydrogen, alkyl of 1 to 6 carbon atoms, chloro or cyano, preferably hydrogen;
n is an integer of 1 or more;
and each q is an integer of 0 to 3.
The preferred organosiloxane bridged bisbenzocyclobutene monomers can be prepared by methods disclosed for example in U.S. Pat. Nos. 4,812,588; 5,136,069; 5,138,081 and WO 94/25903.
According to the most preferred embodiment, the monomer (b), has pendant group acid functionality, and preferably has the formula: 
wherein
B2 is a monovalent organic group with acid functionality, preferably also containing ethylenic unsaturation;
Ar2 is a polyvalent aromatic or heteroaromatic group and the carbon atoms of the cyclobutane ring are bonded to adjacent carbon atoms on the same aromatic ring of Ar2, preferably Ar2 is a single aromatic ring;
is an integer of 1 or more, preferably 1;
R2 is a monovalent group, preferably hydrogen, lower alkyl of up to 6 carbon atoms.
Preferably, monomer (b) is selected from the following two formulas 
wherein
Rxe2x80x2 and Rxe2x80x3 are independently selected from hydrogen, alkyl groups of 1 to 6 carbon atoms, aryl groups, or Rxe2x80x2 and Rxe2x80x3 taken together from a cyclic group of 4 to 8 carbon atoms;
Z is a carbon to carbon bond or an aryl group;
x is an integer from 0 to 3, preferably 1; or 
xe2x80x83wherein
Rxe2x80x2 and Rxe2x80x3 are as defined above,
x is 1, y is 0 or 1, and
Z2 is an aryl group.
The monomer (b) may generally be synthesized by well known Heck chemistryxe2x80x94that is, a Palladium catalyzed coupling of a halogenated-cyclobutarene with a vinyl functional hydroxy containing compound. See for example, U.S. Pat. No. 5,243,068. However, if x=0, monomer (b) may be synthesized by a Grignard reaction of a benzocyclobutene with carbon dioxide or by carbonylation of benzocyclobutene followed by hydrolysis. See U.S. Pat. No. 5,277,536. If y=0, monomer (b) may be synthesized by heating a halogenated cyclobutene with sodium hydroxide.
The preferred ethylenically unsaturated acid functional cyclobutarene may be synthesized by the following reaction as exemplified by reaction with acrylic acid. 
Alternatively, a monomer (bxe2x80x2) may be used. Monomer (bxe2x80x2) is characterized by the presence of a cyclobutarene group and a pendant functionality which at least partially reacts during polymerization of monomers (a) and (bxe2x80x2) to form a pendant acid group. Preferably, monomer (bxe2x80x2) has the formula: 
wherein Rxcex5 and R∀ Rxe2x80x2 and Rxe2x80x3 are as defined above and are preferably hydrogen. When (bxe2x80x2) reacts with (a) the resulting polymer has both acrylate ester and acrylic acid functionalities. Monomer (bxe2x80x2) can be prepared by a palladium-catalyzed arylation reaction of bromobenzocyclobutene.
Monomers (a) and (b) (or (a) and (bxe2x80x2)) are partially polymerized, preferably in solvent but optionally neat, to form an oligomer or prepolymer that may then be used in aqueous base developed imaging systems such as wet etch and photosensitive systems. Polymerization preferably occurs at a temperature in the range of about 125 to about 300xc2x0 C., more preferably about 130 to about 200xc2x0 C. The polymerization may occur for a time determined to provide a partially polymerized resin that provides the desired finally cured film properties. Preferably, the curable product has an apparent weight average molecular weight (Mw) as determined by Gas Permeation Chromatography (GPC) in the range of about 1000 to about 50,000 g/mol, preferably about 1500 to about 25,000 g/mol, and most preferably about 2000 to about 15,000 g/mol. If the molecular weight is too high, development in aqueous base may become difficult.
The molar ratio of monomer (a) to monomer (b) is preferably from about 20:80 to about 70:30, more preferably about 25:75 to about 50:50, and most preferably about 25:75 to about 40:60. Suitable solvents include those that dissolve the reacting monomers at the relevant processing temperature. Preferably, the solvent also dissolves the partially polymerized resin. Examples of such solvents include aromatic hydrocarbons such as toluene, xylene and mesitylene; C3-C6 alcohols; methylcyclohexanone; N-methylpyrrolidinone; butyrolactone; dipropylene glycol dimethyl ether isomers (commercially available from The Dow Chemical Company as Proglyde(trademark) DMM). Dowanol(trademark) DPMA ((di(propylene glycol) methyl ether acetate isomers available from The Dow Chemical Company) is preferred.
While not wishing to be bound by theory, the partially polymerized materials may be formed by a reaction mechanism such as the following example for DVS-bisBCB and BCB-acrylic acid: 
See also Kirchoff and Bruza, Progress in Polymer Science, 18, p. 85 and following. (1993); Farona, Benzocyclocutenes in Polymer Chemistry, Progress in Polymer Science, 21, p. 505 and following (1996); Marks et al., BCB Homopolymerization Chemistry and Applications, The Polymeric Materials Encyclopedia, Salamone, ed., CRC Press, June 1996; and Hahn et al., Thermal Polymerization of Bis(benzocyclobutene)Monomers containing alpha, beta-disubstituted Ethenes, Macromolecules, 26, 15, pp. 3870-3877, 1993.
Optionally, a photoactive composition may be added to the curable polymer to allow the curable polymer to form a photoreactive polymer composition that may be used in photosensitive imaging methods. For negative systems, the photosensitive composition generally comprises a photoactive compound which is a photoinitiator that initiates further cure or cross-linking of the curable polymer. Examples of such compounds include peroxides, azo compounds and benzoin derivatives. See, for example, Photoreactive Polymers; Wiley-Interscience Publication; 1989, pp. 102-127. When exposed to activating radiation, the exposed portions of the composition become insoluble to the developer. Examples of suitable developers include 1,3,5-triisopropylbenzene, decalin and Stoddard reagent. See for example, Dissolution Properties of Photobenzocyclobutene Films, Proceedings of the 10th International Conference on Photopolymers, October-November, 1994.
Preferably, the photosensitive composition comprises a compound which serves as a dissolution inhibitor. This yields a positive photopolymer composition. When the photopolymer composition is exposed to activating radiation, the dissolution inhibitor ceases to function. When the developer solution is applied to the layer of the exposed photopolymer composition, those portions that were exposed to activating radiation are removed. The photosensitive composition may comprise a photoactive compound and a separate dissolution inhibitor or, preferably the dissolution inhibitor may itself be photoactive. Non-limiting examples of suitable photosensitive, dissolution inhibiting compositions/compounds include sulfonyl esters of trihydroxybenzophenone (for example, THBP) and cumyl phenol. See Photoreactive Polymers; Wiley-Interscience Publication; 1989, p. 187. The developer solution is preferably an aqueous base solution. The preferred pH is in the range of 12 to 14. Examples of suitable developers include NaOH, LiOH, KOH, RbOH, CsOH, Me4NOH, Et4NOH. See for example, Photoreactive Polymers, Wiley-Interscience Publication, 1989, p. 216.
Thus, according to one embodiment, the invention is a process for providing an image of a dielectric material comprising the steps of:
a) providing a photosensitive polymer composition comprising
(i) a curable cyclobutarene based polymer comprising acid functional pendant groups as described above and
(ii) a photoactive composition, which preferably comprises a dissolution inhibitor;
b) coating the photosensitive polymer composition onto a substrate;
c) image-wise exposing the coated composition to activating radiation to create a latent image;
d) developing the latent image with a developer, preferably an aqueous base solution.
The substrate preferably may be any substrate known to be useful in the microelectronics industry, such as, for example, silicon, glass, copper, aluminum ceramic, FR4 (glass reinforced epoxy), polyimide, bistriazine, and silicon nitride.
The polymers of this invention are also useful in a wet etch imaging process. That process comprises the steps of:
a) applying a coating of a curable cyclobutarene based polymer comprising acid functional pendant groups as described above to a substrate;
b) applying a mask over the coating,
c) removing portions of the coating not covered by the mask with a developer solution, preferably an aqueous base solution, and
d) removing the mask.
Preferably the mask is a photoresist. The mask is formed by coating the photoresist material onto the coating, exposing the coated photoresist material to activating radiation, and developing the mask and removing the cyclobutarene where portions of the mask are removed. Suitable photoresist materials include Microposit(trademark) 1650 or Microposit(trademark) 1827 (novolac diazoquinone resist from Shipley Co.) and Micropositive Resist 809 from Kodak. Suitable developers for those photoresist materials include the aqueous bases mentioned above.
The polymer of this invention may be coated by any known method including spin-coating, curtain-coating, roller-coating, spray-coating, dip-coating, extrusion coating, meniscus coating and electrodeposition.